All Year 9 children at Burton Borough School in Newport have been told to study from home all of this week after an outbreak involving several students. Four teachers were also advised to isolate.
School staff have been meeting with Public Health England and the local Health Protection Hub to investigate the cause of the outbreak.
An update to families from principal Krissi Carter on Thursday night said: "It is believed that the outbreak has been caused by a family member who returned from international travel and despite a household member having Covid symptoms, the household did not isolate as they should have done and the child who attends Burton Borough School was sent in.
"With any outbreak, the situation will not have concluded yet despite us not having a case since Monday this week – the BBS Covid Rapid Response Team are on standby.
"Public Health England have said that if you or your child does not 'feel right', please isolate and get a PCR test straight away – there should be zero threshold on this.
"If anyone has any symptoms, they must book themselves for a PCR test at a testing site. A lateral flow 30-minute test is not a suitable alternative.
"We also had a family send their children in today [Thursday] despite another household member having symptoms yesterday. I would like to reiterate that if anyone in your household has symptoms you must not send your child/children to school and your whole household must isolate.
"Sending your child/children in is putting everybody at risk, not just at Burton Borough but also in the wider community. As a result of one family not following the guidance, nearly 230 students and their families in Year 9 have been affected and inconvenienced.
"The students of the four teachers who also had to isolate have lost out on face to face teaching."
The school has a dedicated Covid email address for reporting positive tests, at email@example.com